In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is burned, and the hot combustion gases are passed through a turbine mounted on the same shaft. The flow of combustion gas turns the turbine by impingement against an airfoil section of the turbine blades and vanes, which turns the shaft and provides power to the compressor and fan. In a more complex version of the gas turbine engine, the compressor and a high pressure turbine are mounted on one shaft, and the fan and low pressure turbine are mounted on a separate shaft. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forward.
According to thermodynamic principles, the hotter the combustion gases and the exhaust gases, the greater the thermodynamic efficiency of the gas turbine engine. There is an incentive to increase the temperature of the combustion gas. The combustion-gas temperature cannot be raised to an arbitrarily high value, because of the operating temperature limits on the materials of construction of the gas turbine engine.
To allow the combustion and exhaust gas temperatures to be raised as high as possible, several materials and design innovations have been made. The superalloy materials themselves have been improved. The materials in the hottest portions of the gas turbine engine are now made by casting, rather than a wrought process. Single-crystal and oriented-crystal casting is employed.
In another important advance, high-temperature components such as turbine blades for aircraft gas turbines are made hollow so that a flow of cooling air may be directed through the interior of the hollow component during operation. The cooling air flows through the interior and then escapes through openings in the surface of the component at carefully selected locations. The cooling air reduces the temperature of the metal, and allows the combustion gas to be at a higher temperature. The hollow component also has a reduced weight compared to a solid component, an important consideration for any aircraft component but particularly for rotating components.
The hollow turbine blade is typically made by placing a casting core inside a larger-size die, and injecting wax into the space between the casting core and the die. The die is removed, and a ceramic-shell casting mold is formed over the wax. The wax is removed, leaving a casting space between the casting core and the ceramic-shell casting mold. Molten casting metal is poured into the casting space between the casting core and the ceramic-shell casting mold.
The casting core is prevented from touching the inner wall of the ceramic shell by standoff spacers extending between the two. These standoff spacers may, however, undesirably extend through the wall of the completed hollow airfoil, leaving a hole therethrough. Any cooling air that flows through such through-holes may reduce the cooling efficiency and the overall performance of the hollow component, if that cooling air flow out of the hollow component is not at the carefully selected locations that maximize the effect of the cooling air in improving performance. The through-holes also potentially compromise the mechanical performance of the article, by providing a source of weakness and possible premature failure.
Several approaches have been used to deal with this problem. In one, the through-holes are ignored, and the loss in cooling efficiency is accepted. In another, larger through-holes are plugged, and smaller through-holes are ignored. This achieves a partial solution. The plugging techniques typically involve drilling out the through-hole to a standard size, inserting a freestanding plug into the through-hole or placing a freestanding platelet closure over the through-hole, and then welding the plug or closure in place. This approach is difficult to apply to smaller-size through-holes, due to the amount of labor involved.
These approaches either achieve only a partial solution, or the solution is expensive and laborious. Mechanical properties are often compromised, because the welding of the inserted plug or platelet may leave a heat-affected zone that cannot be properly heat treated.
Accordingly, there is a need for an improved approach to the sealing of such through-holes. The present invention fulfills this need, and further provides related advantages.